MRAM wet etch method

ABSTRACT

An etching process is employed to selectively pattern the top magnetic film layer, the tunnel barrier, and the pinned bottom magnetic layer of a magnetic thin film structure. The pinned bottom magnetic film layer has an antiferromagnetic layer or a Ru spacer formed thereunder. The etching process employs various etching steps that selectively remove various layers of the magnetic thin film structure stopping on the antiferromagnetic layer or the Ru spacer. The progress of this etching process can be monitored by measuring the electrochemical potential difference of a part or wafer containing a magnetic structure with respect to a reference electrode simultaneously with the selective etching process.

RELATED APPLICATIONS

This application is a divisional application of U.S. Ser. No.11/147,513, filed Jun. 8, 2005.

This application was sponsored by the United States Government underContract No. MDA 972-99-C-0009, which was awarded by the DefenseAdvanced Research Projects Agency (DARPA); therefore, the United StatesGovernment has certain rights and privileges in the present application.

FIELD OF THE INVENTION

The present invention relates to the processing of magnetic thin filmstructures, and in particular to an etching process for selectivelyetching the exposed top magnetic layer, and the underlying tunnelbarrier and pinned bottom magnetic layer of a magnetic random accessmemory (MRAM) stack.

BACKGROUND OF THE INVENTION

MRAM has tremendous potential as a nonvolatile, solid-state memory toreplace flash memory and electronically erasable programmable read-onlymemory (EEPROM). To enhance the performance of MRAM chips, it isnecessary to reduce the lithographic minimum printable size. However,ion beam etching (IBE), a.k.a. ion milling, has been essentially theonly method available for creating fine patterns, e.g., submicronpatterns, in magnetic thin film structures. Because of the lack ofvolatile compounds for ferrous metals other than carbonyl, reactive-ionetching (RIE) has not been a viable technique for patterning thinmagnetic films; and a RIE process based on carbonyl chemistry has notyet been developed. Thus, a chemical etching technique for patterningmagnetic thin films based on Fe, Co and Ni is attractive because of thethin film nature of MRAM magnetic films (20-50 Å z-direction) relativeto the x-y dimensions of patterned magnetic tunnel junction (MTJ)elements. The MRAM structure represents a complex multilayer systemwhich includes numerous magnetic thin film layers. A typical MRAMstructure is shown in FIG. 1. Specifically, the thin film structureshown in FIG. 1 comprises Si substrate 10, SiO_(x) layer 12, a 150 Å Tilayer 14, Ni₈₁Fe₁₉ (40 Å) layer 16, Ir₂₀Mn₈₀ (120 Å) layer 18, CO₉₀Fe₁₀(20 Å) layer 20, Al₂O₃ (10 Å) layer 22, Ni₈₁Fe₁₉ (40 Å) layer 24 and Ti(100 Å) layer 26. In this prior art magnetic structure, Al₂O₃ layer 22serves as a tunnel barrier between the top magnetic film layer, i.e.,Ni₈₁Fe₁₉ layer 24, and the pinned bottom magnetic layer, i.e., Co₉₀Fe₁₀layer 20, the antiferromagnetic layer 18 and the magnetic layer 16 whichare present beneath the tunnel barrier layer 22. Layer 26 is apassivating layer that prevents moisture, air or other contaminants fromentering into the structure, while layer 14 is an adhesion layer. In thecase of AP-pinned MTJs, the antiferromagnetic layer 18 can be a Ruspacer. The term “AP-pinned MTJS” is used herein to denote MTJs whichcontain an antiparallel-pinned (AP) layer structure, wherein the APlayer structure includes at least two pinned layers having magneticmoments that are self-pinned antiparallel to each other and the at leasttwo pinned layers are separated by an AP coupling layer or a spacer.

As is well known to those skilled in the art, the magnetic films of aMRAM structure, such as illustrated in FIG. 1, are quite thin.Patterning of prior art MRAM structures, such as shown in FIG. 1, istypically carried out by first applying a mask to the MRAM structure andpatterning the mask by lithography (exposure and development). FIG. 2shows the structure after these steps wherein reference number 28represents the patterned mask. The pattern is transferred to the MRAMstructure by RIE, IBE, or wet etching. And then the exposed Ni₈₁Fe₁₉ (40Å) layer 24 can be pattern-wise etched by RIE, IBE, or wet etching. In atraditional wet etching process, a standard aqueous acid solution, suchas sulfuric and/or nitric acid, is employed to etch the exposed Ni₁₈Fe₁₉(40 Å) layer 24. Although the acid etchants are capable of etchingthrough the exposed top magnetic layer 24 of the structure, the acidetchants are not selective for removing just that magnetic layer 24.Instead, when the acid etchants are employed, they also etch theunderlying alumina tunnel barrier layer 22, the pinned Co₉₀Fe₁₀ layer20, and the Mn in the Ir₂₀Mn₈₀ layer 18 of the magnetic thin film stackproviding the structure shown in FIG. 3.

Despite being capable of etching numerous magnetic layers in a MRAMstructure, the use of prior art aqueous acid solutions causesGalvanic-coupling-accelerated dissolution of the Co₉₀Fe₁₀ (20 Å) layer20 which is coupled to an antiferromagnetic layer in simple-pinned MTJsand to a Ru spacer in AP-pinned MTJs. The term “simple-pinned MTJs” isused herein to denote MTJs which contain a single reference layer,wherein the single reference layer has its magnetization typicallypinned by exchange coupling with an antiferromagnetic layer. In MRAM,due to coupled active magnetic layers and noble metals, avoidance ofGalvanically-accelerated dissolution is a major concern.

U.S. Pat. No. 6,426,012 describes a method to pattern the magnetic softlayer of a MRAM structure while avoiding Galvanically-enhanced etchingreactions. However, the carboxylic acids employed in U.S. Pat. No.6,426,012 are weak acids, and thus are not capable of etching throughthe alumina tunnel barrier 22.

A desirable situation would be to selectively etch through the exposedtop magnetic layer, i.e., layer 24, as well as the underlying thin Al₂O₃layer 22, and the pinned bottom magnetic layer, i.e., layer 20, in theMRAM structure, and stop the etch process at the antiferromagnetic layer18, while avoiding Galvanically-enhanced etching reactions. In the caseof AP-pinned MTJs, the etching process stops at the Ru spacer of theAP-pinned layer structure. Such a method would leave theantiferromagnetic layer, in the case of simple-pinned MTJs, and the Ruspacer, in the case of AP-pinned MTJs, unetched.

To date, applicants are unaware of any etching process which selectivelyetches a magnetic thin film structure so as to stop on theantiferromagnetic layer, in the case of simple-pinned MTJs, or on the Ruspacer, in the case of AP-pinned MTJs, while avoiding Galvanic corrosionor Galvanically-assisted lateral etching of the edges of exposedmagnetic layers. There is thus a need for developing an etching processwhich is capable of selectively etching a magnetic thin film structureto provide a patterned structure wherein the pattern is not formed inthe antiferromagnetic layer, in the case of simple-pinned MTJs, or inthe Ru spacer, in the case of AP-pinned MTJs. Such an etching processwould be beneficial since it would prevent unwanted Galvanic corrosionof the inner magnetic layers, while being able to pattern the topmagnetic film layer, the tunnel barrier layer, and the pinned bottommagnetic layer of the structure. The ability to etch down to an etchstop layer such as the antiferromagnetic layer or the Ru spacer furtherhas advantages of superior process repeatability and avoiding thedifficulties associated with stopping the etch process at alumina andleaving intact the pinned magnetic layer.

SUMMARY OF THE INVENTION

The present intention is directed to a method of selectively patterningthe exposed top magnetic film layer, and the underlying tunnel barrierand the pinned bottom magnetic layer of a magnetic structure, stoppingon the antiferromagnetic layer, in the case of simple-pinned MTJs, or onthe Ru spacer, in the case of AP-pinned MTJs, in which the variousetching processes employed do not adversely damage the antiferromagneticlayer or the Ru spacer that are present beneath the pinned bottommagnetic layer. The aforementioned object is achieved by utilizing aselective etching process. Specifically, the above object is obtainableutilizing processing steps that include:

-   -   (a) providing a magnetic structure comprising at least one        pinned bottom magnetic film layer and at least one top magnetic        film layer, wherein said at least one top and said at least one        pinned bottom magnetic film layers are separated by a tunnel        barrier layer, and said at least one top magnetic film layer has        a passivating layer located thereon;    -   (b) forming a patterned resist atop said passivating layer,        wherein a portion of said passivating layer is exposed;    -   (c) selectively etching said exposed portion of said passivating        layer by a RIE process to expose a portion of said at least one        top magnetic film layer; and    -   (d) selectively etching said exposed portion of said at least        one top magnetic film layer, and underlying portions of said        tunnel barrier and said at least one pinned bottom magnetic film        layer by a wet etch process which includes an etchant solution        comprising an arylsulfonic acid and an aliphatic or alicyclic        amine.

The present intention is also directed to a method of monitoring theprogress of a selective etching process and determining the etchendpoint of said selective etching process, which comprises measuringthe electrochemical potential difference of a part or wafer containing amagnetic structure with respect to a reference electrode simultaneouslywith the selective etching process.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-3 are pictorial views through cross-sections illustrating atypical prior art process of patterning a magnetic thin film structure.

FIGS. 4-8 are pictorial views through cross-sections illustrating thebasic processing steps of the present invention which are used inpatterning a magnetic thin film structure.

FIG. 9 is a schematic illustration of a typical wet etching experimentalsetup that can be employed in the present invention.

FIG. 10 is a graph plotting substrate potential (mV v. Ag/AgCl) v. time(minute) illustrating a typical Open Circuit Potential (E_(oc)) v. Timecurve of one embodiment of the present invention.

DESCRIPTION OF THE INVENTION

The present invention, which provides a method for selectively etchingthe top magnetic layer, the tunnel barrier, and the pinned bottommagnetic layer of a magnetic thin film structure, without adverselyaffecting the antiferromagnetic layer, in the case of simple-pinnedMTJs, or the Ru spacer, in the case of AP-pinned MTJs, will now bedescribed in more detail by referring to the drawings that accompany thepresent application. It is noted that the drawings of the presentinvention are provided for illustrative purposes and thus they are notdrawn to scale.

Reference is first made to FIG. 4 which includes a portion of a magneticthin film structure that can be utilized in the present invention.Specifically, the portion of the magnetic structure shown in FIG. 4includes at least one pinned bottom magnetic thin film layer 52 and atleast one top magnetic film layer 56 which are separated by a tunnelbarrier layer 54. The upper most surface layer of the at least one topmagnetic layer 56 has a passivating layer 58 formed thereon and thepassivating layer 58 includes a surface oxide 60. In the case ofsimple-pinned MTJs, the at least one pinned bottom magnetic thin filmlayer 52 has an antiferromagnetic layer 50 formed thereunder. In thecase of AP-pinned MTJs, the at least one pinned bottom magnetic thinfilm layer 52 has a Ru spacer 50 formed thereunder. The portion of themagnetic thin film structure shown in FIG. 4 may further include asemiconductor substrate, a SiO_(x) layer, a Ti layer, another magneticfilm layer, or other adhesion layers which would be located beneath theantiferromagnetic layer 50, in the case of simple-pinned MTJs, or the Ruspacer 50, in the case of AP-pinned MTJs. When these elements arepresent, the magnetic thin film structure would look similar to the onedepicted in FIG. 1. For clarity, however, the material layers locatedbeneath the antiferromagnetic layer 50, in the case of simple-pinnedMTJs, or the Ru spacer 50, in the case of AP-pinned MTJs, are not shownin FIGS. 4-8.

The portion of the magnetic thin film structure illustrated in FIG. 4 isformed utilizing conventional techniques well known in the art that arecapable of forming such a structure. For example, the various magneticand non-magnetic layers may be formed by utilizing the same or differentdeposition process including, for example, chemical vapor deposition(CVD), plasma-assisted CVD, plating, evaporation, sputtering, chemicalsolution deposition and other like deposition processes.

The top and pinned bottom magnetic film layers (56 and 52, respectively)of the illustrated structure may be composed of the same or differentmagnetic layers in which each individual layer has a thickness of lessthan 150 Å. Preferably, each individual magnetic film layer (52 or 56)has a thickness of less than 100 Å.

Illustrative examples of magnetic films that can be employed in thepresent invention as layers 52 or 56 include, but are not limited to:Ni_(x)Fe_(y), Ni_(x)Co_(y)Fe_(z) and other like magnetic films. In theabove formulas, x, y, and z are any integers whose sum adds up to 100.In accordance with the present invention, the pinned bottom magneticfilm layer 52 as well as the top magnetic film layer 56 may compriseonly one magnetic film or they may comprise a stack of magnetic films.

In one embodiment of the present invention, the top magnetic film layer56 is comprised of a Permalloy layer, i.e., a Ni_(x)Fe_(y), orCo_(x)Fe_(y) layer, while the bottom magnetic film layer 52 comprises astack consisting of Ni_(x)Fe_(y) and Co_(x)Fe_(y).

The antiferromagnetic layer 50 may be composed of H_(x)Mn_(y), wherein His a noble metal and x, and y are any integers whose sum adds up to 100.

Illustrative examples of antiferromagnetic layers 50 that can beemployed in the present invention include, but are not limited to:Ir₂₀Mn₈₀ or Pt₅₀Mn₅₀ and other like noble metal-containingantiferromagnetic layers.

In one embodiment of the present invention, any exposed edge of themagnetic structure may be coated with a conventional passivatingmaterial so as to prevent certain underlying films, notably the noblemetal containing exchange bias layer from engaging in Galvanic typereactions that could inhibit the etching process.

The tunnel barrier layer 54 employed in the present invention includesany conventional material layer such as Al₂O₃ which is capable ofsustaining a tunneling current and which does not chemically degrade theproperties of the top and pinned bottom magnetic layers (56 and 52,respectively). In some instances, the tunnel barrier layer 54 may alsoserve as a diffusion barrier. The tunnel barrier layer 54 employed inthe present invention is a thin layer which typically has a thickness ofless than about 15 Å.

The passivating layer 58 employed in the present invention includes anymetal layer such as Ti, TiN, Ta, or TaN which serves as a barrier layerpreventing diffusion of moisture, air and other contaminants fromcontacting with the underlying magnetic layers and the tunnel barrierlayer 54. The thickness of this layer may vary, but typically thepassivating layer 58 has a thickness from about 20 to about 1000 Å.

As indicated above, the passivating layer 58 contains a surface oxide 60which is located on the upper surface of the passivating layer 58 whenthe structure is exposed to air.

Next, and as illustrated in FIG. 5, patterned resist 62 is formed on thesurface oxide 60 present on the passivating layer 58 as shown in FIG. 4utilizing conventional lithography. Specifically, the lithographyprocess includes: (i) applying a conventional resist to the surfaceoxide 60 of the passivating layer 58 via a conventional depositionprocess such as spin-on coating, dip coating, CVD, plasma-assisted CVD,evaporation and chemical solution deposition; (ii) exposing the resistto radiation to form a desired pattern therein; and (III) developing thedesired pattern using a conventional developer solution to expose aportion of the surface oxide 60 on the passivating layer 58.

Following the formation of the patterned resist, the inventive etchingprocess, which will be described in more detail below, is performed.First, the exposed surface oxide 60, not protected with the patternedresist 62, is removed by utilizing a reactive-ion etching (RIE) processproviding the structure shown FIG. 6. The RIE process may include CF₄/Aror CBrF₃/SF₆ as etchant gases. Any suitable RIE process may be used,such as the process disclosed in U.S. Pat. No. 6,426,012; the entirecontent of which is incorporated herein by reference. Those skilled inthe art can ascertain the suitable conditions without undue experiments.As is illustrated, this etching step exposes a portion of thepassivating layer 58.

In one embodiment of the present invention, an optional oxygen ashingprocess is performed prior to removing the exposed surface oxide 60. Theoptional oxygen ashing process includes the use of oxygen ashingconditions well known to those skilled in the art that are capable ofremoving any etched resist residue from the exposed surface of thesurface oxide 60 of the passivating layer 58. In another embodiment ofthe present invention, a conventional oxygen ashing process which iscapable of removing any fluorine-related resist residue may be performedafter conducting the above described RIE steps.

Next, the exposed portion of the passivating layer 58 that does notinclude any inert surface oxide layer is subjected to a suitable etchingprocess that includes the use of RIE process. Any suitable etchingprocess may be used, such as the etching process disclosed in U.S. Pat.No. 6,426,012. Those ordinarily skilled in the art can ascertain thesuitable conditions without undue experiments. In this step of thepresent invention, the passivating layer 58 is selectively patterned soas to expose a portion of the at least one top magnetic film layer 56,See FIG. 7.

Alternatively, and when a Ta (or TaN) passivating layer 58 is employed,the above wet etching step may be replaced with a SF₆ RIE plasma etchingprocess. The use of SF₆ RIE plasma etching to remove the Ta (or TaN)passivating layer 58 also modifies the surface of the exposed magneticlayer 56 to include sulfur. Alternatively, another RIE etchant gas whichremoves Ta (or TaN) could be used first and thereafter the etchedsurface is treated with a SF₆ gas.

The presence of sulfur on a magnetic thin film layer 56 is advantageoussince sulfur-containing magnetic film layers etch at a much faster ratethan magnetic film layers which does not include sulfur fragments.

FIG. 8 shows the next step of the present invention in which exposed topmagnetic layer 58, as well as the underlying tunnel barrier 54, and thebottom magnetic layer 52, are selectively etched utilizing an etchingprocess that is capable of stopping on the antiferromagnetic layer 50,in the case of simple-pinned MTJs, or on the Ru spacer 50, in the caseof AP-pinned MTJs. In this step of the present invention, the exposedtop magnetic film layer 56, the underlying tunnel barrier 54, and thebottom magnetic layer 52, are selectively etched utilizing the inventiveetchant solution that comprises an arylsulfonic acid or a salt thereofand an aliphatic or alicyclic amine. By “an arylsulfonic acid or a saltthereof”, it is meant a sulfonic acid derivative that is substitutedwith an aryl group or any metal salt of such an arylsulfonic acid. By“an aliphatic amine” it is meant an amine derivative that is substitutedwith a hydrocarbon compound having an open-chain structure. By “analicyclic amine” it is meant an amine derivative that is substitutedwith a hydrocarbon compound that contains a ring, but is not aromatic.In a preferred embodiment of the present invention, the salt thereof isan alkali metal salt of the arylsulfonic acid.

The arylsulfonic acids suitable for the present invention includenitrobenzene sulfonic acids having one of the following structures:

wherein R1, R2, R3, and R4 are the same or different, and areindependently hydrogen, C₁-C₃ alkyl, halogen, amino, or hydroxyl group.

Illustrative examples of arylsulfonic acids and salts thereof that canbe employed in the present invention include, but are not limited to:3-nitrobenzene sulfonic acid, a.k.a. m-nitrobenzene sulfonic acid;4-chloro-3-nitrobenzene sulfonic acid; sodium 3-nitrobenzene sulfonate;4-nitrobenzene sulfonic acid; 2-methyl-5-nitrobenzene sulfonic acid;2-amino-4-nitrophenol-6-sulfonic acid; 2-nitrobenzene sulfonic acid;2-chloro-5-nitrobenzene sulfonic acid; 3-amino-4-hydroxy-5-nitrobenzenesulfonic acid; and 3-amino-2-hydroxy-5-nitrobenzene sulfonic acid. In apreferred embodiment of the present invention, the arylsulfonic acid ism-nitrobenzene sulfonic acid. In one embodiment of the inventive etchantsolution, the arylsulfonic acid is m-nitrobenzenesulfonic acid (NBSA).

The aliphatic or alicyclic amines suitable for the present inventioninclude primary, secondary, and tertiary amines. Illustrative examplesof aliphatic or alicyclic amines that can be employed in the presentinvention include, but are not limited to: propylamine, ethylenediamine,diethylenetriamine, triethylenetetramine and other analogous amines. Itis understood to one skilled in the art that triethylenetetramine can belinear, branched, cyclic, or a mixture thereof. In one embodiment of theinventive etchant solution, the aliphatic or alicyclic amine isethylenediamine.

The inventive etchant solution may further comprise dissolved O₂. Thedissolved O₂ is at an equilibrium concentration due to air atmosphere.The inventive etchant solution includes a mole ratio of the arylsulfonicacid to the aliphatic or alicyclic amine from about 1:3 to about 1:20,with a mole ratio of the arylsulfonic acid to the aliphatic or alicyclicamine from about 1:6 to about 1:10 being more preferred. The etchantsolution typically comprises from about 0.1 to m parts by weight of anarylsulfonic acid per 100 parts by weight water where m is limited bythe solubility of the acid. Typically, the pH of the inventive etchantsolution is from about 5 to about 10, with the pH of about 6.5 to about8.5 more preferred. In one embodiment of the present invention, the pHof the inventive etchant solution is about 7.5.

This etching step is typically carried out at temperature of about 40°to about 50° C. for a period of time from about 3 to about 12 minutes,depending on the source and type of parts, the condition of the free topmagnetic layer 56, and the thickness of the tunnel barrier layer 54.

The progress of a selective etching process, such as the above etchstep, may be monitored by measuring the electrochemical potentialdifference of a part or wafer containing a magnetic structure withrespect to a reference electrode simultaneously with the selectiveetching process. FIG. 9 shows a typical experimental setup of theinventive wet etching process. A typical reference electrode is Ag/AgClelectrode. In one embodiment, the etch tank is a thermostatted Jacketedpolymer tank for 200 mm substrates. In another embodiment, the etch tankis a thermostatted jacketed Pyrex tank with magnetic stirring forpreparing conducting atomic force microscopy (CAFM) parts. The etchantsolution is preferably pre-filtered using 0.2 μm filter.

The etch endpoint of the selective etching process, such as the aboveetch step, may be determined by measuring the electrochemical potentialdifference of a part or wafer containing a magnetic structure withrespect to a reference electrode simultaneously with the selectiveetching process. FIG. 10 shows the curve of substrate potential v. etchtime for one embodiment of simple pinned MTJs etched inNBSA/ethylenediamine solution at 45° C. The curve exhibits a steadydecrease in substrate potential when the Ni₈₁Fe₁₉ top layer isdissolved, and reaches the bottom when the AlO_(x) layer is dissolved.While dissolution of Co_(x)Fe_(y) layer is underway, the curve exhibitsa rapid rise in substrate potential. When Pt₅₀Mn₅₀ layer is exposed, theoblique curve becomes a plateau in substrate potential. Thecharacteristic of exposed Pt₅₀Mn₅₀ is the etch endpoint of this etchingprocess.

The application of the novel electrochemical monitoring technique fordetermining etch endpoint enables the inventive wet etch method can beused for preparing samples for routine CAFM screening tests, and theability of the inventive wet etch method to etch past the tunnel barrierand minimize or eliminate Galvanic corrosion effects are consistentlydemonstrated.

Following the third etching step of the present invention, the patternedstructure may be rinsed with a suitable inert solvent or supercriticalfluid and the patterned resist may then be stripped utilizing any wellknown stripping process. The patterned magnetic structure may then besubjected to conventional MRAM processing techniques also well known tothose skilled in the art.

While this invention has been particularly shown and described withrespect to preferred embodiments thereof it will be understood by thoseskilled in the art that the foregoing and other changes in forms anddetails may be made without departing from the spirit and scope of thepresent invention. It is therefore intended that the present inventionnot be limited to the exact forms and details described and illustrated,but all within the scope of the appended claims.

1. A method of monitoring the progress of a selective etching processand determining the etch endpoint of said selective etching processcomprising measuring the electrochemical potential difference of a partor wafer containing a magnetic structure with respect to a referenceelectrode simultaneously with the selective etching process, whereinsaid magnetic structure comprises at least one pinned bottom magneticfilm layer and at least one top magnetic film layer, wherein said atleast one top and said at least one pinned bottom magnetic film layersare separated by a tunnel barrier layer, and said at least one topmagnetic film layer having a passivating layer located thereon.
 2. Themethod of claim 1 wherein said reference electrode is Ag/AgCl.
 3. Themethod of claim 1 wherein said selective etching process is conducted ata temperature of about 40°-50° C.
 4. The method of claim 1 wherein saidat least one top magnetic film layer and said at least one pinned bottommagnetic film layer are the same or different, and are Ni_(x)Fe_(y),Ni_(x)Co_(y)Fe_(z), or Co_(x)Fe_(y) wherein x, y and z are any integerswhose sum adds up to
 100. 5. The method of claim 4 wherein said at leastone top magnetic layer is Ni_(x)Fe_(y) and the at least one pinnedbottom magnetic film layer is Ni_(x)Fe_(y), or Co_(x)Fe_(y).
 6. Themethod of claim 3 wherein said at least one pinned bottom magnetic filmlayer has an antiferromagnetic layer formed thereunder.
 7. The method ofclaim 6 wherein said antiferromagnetic layer is H_(x)Mn_(y) wherein H isa noble metal and x and y are any integers whose sum adds up to
 100. 8.The method of claim 6 wherein said antiferromagnetic layer isIr_(x)Mn_(y), or Pt_(x)Mn_(y) wherein x and y are any integers whose sumadds up to
 100. 9. The method of claim 1 wherein said at least onepinned bottom magnetic film layer has a Ru spacer formed thereunder. 10.The method of claim 1 wherein said passivating layer is one of Ti, Ta,TiN or TaN.
 11. The method of claim 1 wherein said tunnel barrier layeris Al₂O₃.
 12. The method of claim 1 wherein said selective etchingprocess comprises at least a wet etch which includes an etchant solutionincluding an arylsulfonic acid and an aliphatic or alicyclic amine. 13.The method of claim 12 wherein said arylsulfonic acid in the etchantsolution has one of the following structures:

wherein R1, R2, R3, and R4 are the same or different, and areindependently hydrogen, C₁-C₃ alkyl, halogen, amino, or hydroxyl group.14. The method of claim 12 wherein said arylsulfonic acid or a saltthereof in the etchant solution is selected from the group consisting ofm-nitrobenzene sulfonic acid, 4-chloro-3-nitrobenzene sulfonic acid;sodium 3-nitrobenzene sulfonate; 4-nitrobenzene sulfonic acid;2-methyl-5-nitrobenzene sulfonic acid; 2-amino-4-nitrophenol-6-sulfonicacid; 2-nitrobenzene sulfonic acid; 2-chloro-5-nitrobenzene sulfonicacid; 3-amino-4-hydroxy-5-nitrobenzene sulfonic acid; and3-amino-2-hydroxy-5-nitrobenzene sulfonic acid.
 15. The method of claim12 wherein said aliphatic or alicyclic amine in the etchant solution isselected from the group consisting of propylamine, ethylenediamine,diethylenetriamine, and triethylenetetramine.
 16. The method of claim 12wherein said etchant solution further comprising dissolved O₂, whereinsaid dissolved O₂ is at an equilibrium concentration under airatmosphere.
 17. The method of claim 12 wherein said etchant solution hasa pH of about 5 to about
 10. 18. The method of claim 12 wherein saidetchant solution is pre-filtered.
 19. The method of claim 12 whereinsaid arylsulfonic acid and said aliphatic or alicyclic amine are presentin said etchant in a mole ratio from about 1:3 to about 1:20,respectively.
 20. A method of monitoring the progress of a selectiveetching process and determining the etch endpoint of said selectiveetching process comprising measuring the electrochemical potentialdifference of a part or wafer containing a magnetic structure withrespect to a reference electrode simultaneously with the selectiveetching process, wherein said selective etching process comprises atleast a wet etch which includes an etchant solution including anarylsulfonic acid and an aliphatic or alicyclic amine.